Deep diving is underwater diving to a depth beyond the norm accepted by the associated community. In some cases this is a prescribed limit established by an authority, while in others it is associated with a level of certification or training, and it may vary depending on whether the diving is recreational, technical or commercial. Nitrogen narcosis becomes a hazard below 30 metres (98 ft) and hypoxic breathing gas is required below 60 metres (200 ft) to lessen the risk of oxygen toxicity. At much greater depths, breathing gases become supercritical fluids, making diving with conventional equipment effectively impossible regardless of the physiological effects on the human body. Air, for example, becomes a supercritical fluid below about 400 metres (1,300 ft).
For some recreational diving agencies, "Deep diving", or "Deep diver" may be a certification awarded to divers that have been trained to dive to a specified depth range, generally deeper than 30 metres (98 ft). However, the Professional Association of Diving Instructors (PADI) defines anything from 18 to 30 metres (59 to 98 ft) as a "deep dive" in the context of recreational diving (other diving organisations vary), and considers deep diving a form of technical diving. [1] [ page needed ] In technical diving, a depth below about 60 metres (200 ft) where hypoxic breathing gas becomes necessary to avoid oxygen toxicity may be considered a deep dive. In professional diving, a depth that requires special equipment, procedures, or advanced training may be considered a deep dive.
Deep diving can mean something else in the commercial diving field. For instance early experiments carried out by COMEX using heliox and trimix attained far greater depths than any recreational technical diving. One example being its "Janus 4" open-sea dive to 501 metres (1,640 ft) in 1977. [2] [3]
The open-sea diving depth record was achieved in 1988 by a team of COMEX and French Navy divers who performed pipeline connection exercises at a depth of 534 metres (1,750 ft) in the Mediterranean Sea as part of the "Hydra 8" programme employing heliox and hydrox. The latter avoids the high-pressure nervous syndrome (HPNS) caused by helium and eases breathing due to its lower density. [2] [4] [5] These divers needed to breathe special gas mixtures because they were exposed to very high ambient pressure (more than 54 times atmospheric pressure).
An atmospheric diving suit (ADS) allows very deep dives of up to 700 metres (2,300 ft). [6] These suits are capable of withstanding the pressure at great depth permitting the diver to remain at normal atmospheric pressure. This eliminates the problems associated with breathing pressurised gases. In 2006 Chief Navy Diver Daniel Jackson set a record of 610 metres (2,000 ft) in an ADS. [7] [8]
On 20 November 1992 COMEX's "Hydra 10" experiment simulated a dive in an onshore hyperbaric chamber with hydreliox. Théo Mavrostomos spent two hours at a simulated depth of 701 metres (2,300 ft). [2] [9] [10] [11] [12]
Assumed is the surface of the waterbody to be at or near sea level and underlies atmospheric pressure.
Not included are the differing ranges of freediving – without breathing during a dive.
Depth [nb 1] | Comments |
---|---|
12 m (39 ft) | Recreational diving limit for divers aged under 12 years old and EN 14153-1 / ISO 24801-1 level 1 (Supervised Diver) standard. [13] |
18 m (60 ft) | Recreational diving limit for Open Water Divers (e.g. PADI, NAUI). |
20 m (66 ft) | Recreational diving limit for EN 14153-2 ISO 24801-2 level 2 "Autonomous Diver" standard. [14] |
21 m (69 ft) | GUE Recreational Diver Level 1. [15] |
30 m (98 ft) | Recommended recreational diving limit for PADI Advanced Open Water divers [1] [ page needed ] and GUE Recreational Diver Level 2. [15] Average depth at which nitrogen narcosis symptoms begin to be noticeable in adults. |
40 m (130 ft) | Depth limit for divers specified by Recreational Scuba Training Council [1] [ page needed ] and GUE Recreational Diver Level 3. [15] Depth limit for a French level 2 diver accompanied by an instructor (level 4 diver), breathing air.[ citation needed ] |
50 m (160 ft) | Depth limit for divers breathing air specified by the British Sub-Aqua Club and Sub-Aqua Association. [16] |
60 m (200 ft) | Depth limit for a group of 2 to 3 French Level 3 recreational divers, breathing air. [17] |
66 m (217 ft) | Depth at which breathing compressed air exposes the diver to an oxygen partial pressure of 1.6 bar (23 psi). Greater depth is considered to expose the diver to an unacceptable risk of oxygen toxicity. [nb 2] |
100 m (330 ft) | One of the recommended technical diving limits. Maximum depth authorised for divers who have completed Trimix Diver certification with IANTD [18] or Advanced Trimix Diver certification with TDI. [19] |
156 m (512 ft) | Deepest scuba dive on compressed air (July 1999 in Puerto Galera, Philippines). [20] |
200 m (660 ft) | Limit for surface light penetration sufficient for plant growth in clear water, though some visibility may be possible farther down. [nb 3] |
230 m (750 ft) | First dive on a hydrox-rebreather (14 February 2023 in the Pearse Resurgence, New Zealand). [21] |
290 m (950 ft) | Deepest ocean dive on a rebreather (23 March 2014 in Gili Trawangan, Indonesia). [22] |
312 m (1,024 ft) | Deepest cave diving on a rebreather (6 January 2024 in Font Estramar, France). |
316 m (1,037 ft) | Deepest dive on a rebreather (10 October 2018 in Lake Garda, Italy). [23] |
332 m (1,089 ft) | Deepest scuba dive, deepest dive on trimix (18 September 2014 in Dahab, Egypt). [24] [25] |
534 m (1,752 ft) | COMEX Hydra 8 dives on hydreliox (February 1988 offshore Marseille, France). [2] [4] [10] |
Deep diving has more hazards and greater risk than basic open-water diving. [26] Nitrogen narcosis, the "narks" or "rapture of the deep", starts with feelings of euphoria and over-confidence but then leads to numbness and memory impairment similar to alcohol intoxication. [1] [ page needed ] Decompression sickness, or the "bends", can happen if a diver ascends too rapidly, when excess inert gas leaves solution in the blood and tissues and forms bubbles. These bubbles produce mechanical and biochemical effects that lead to the condition. The onset of symptoms depends on the severity of the tissue gas loading and may develop during ascent in severe cases, but is frequently delayed until after reaching the surface. [1] [ page needed ] Bone degeneration (dysbaric osteonecrosis) is caused by the bubbles forming inside the bones; most commonly the upper arm and the thighs. Deep diving involves a much greater danger of all of these, and presents the additional risk of oxygen toxicity, which may lead to convulsions underwater. Very deep diving using a helium-oxygen mixture (heliox) or a hydrogen-helium-oxygen mixture (hydreliox) carries the risk of high-pressure nervous syndrome and hydrogen narcosis. Coping with the physical and physiological stresses of deep diving requires good physical conditioning. [27]
Using open-circuit scuba equipment, consumption of breathing gas is proportional to ambient pressure – so at 50 metres (164 ft), where the pressure is 6 bars (87 psi), a diver breathes six times as much as on the surface (1 bar, 14.5 psi). Heavy physical exertion makes the diver breathe even more gas, and gas becomes denser requiring increased effort to breathe with depth, leading to increased risk of hypercapnia – an excess of carbon dioxide in the blood. The need to do decompression stops increases with depth. A diver at 6 metres (20 ft) may be able to dive for many hours without needing to do decompression stops. At depths greater than 40 metres (131 ft), a diver may have only a few minutes at the deepest part of the dive before decompression stops are needed. In the event of an emergency, the diver cannot make an immediate ascent to the surface without risking decompression sickness. All of these considerations result in the amount of breathing gas required for deep diving being much greater than for shallow open water diving. The diver needs a disciplined approach to planning and conducting dives to minimise these additional risks.
Many of these problems are avoided by the use of surface supplied breathing gas, closed diving bells, and saturation diving, at the cost of logistical complexity, reduced maneuverability of the diver, and greater expense.
Both equipment and procedures can be adapted to deal with the problems of greater depth. Usually the two are combined, as the procedures must be adapted to suit the equipment, and in some cases the equipment is needed to facilitate the procedures.
The equipment used for deep diving depends on both the depth and the type of diving. Scuba is limited to equipment that can be carried by the diver or is easily deployed by the dive team, while surface-supplied diving equipment can be more extensive, and much of it stays above the water where it is operated by the diving support team.[ citation needed ]
Procedural adaptations for deep diving can be classified as those procedures for operating specialized equipment, and those that apply directly to the problems caused by exposure to high ambient pressures.
Amongst technical divers, there are divers who participate in ultra-deep diving on scuba below 200 metres (656 ft). This practice requires high levels of training, experience, discipline, fitness and surface support. Only twenty-six people are known to have ever dived to at least 240 metres (790 ft) on self-contained breathing apparatus recreationally. [20] [28] [nb 4] [nb 5] The "Holy Grail" of deep scuba diving was the 300 metres (980 ft) mark, first achieved by John Bennett in 2001, and has only been achieved five times since.[ citation needed ] Due to the short bottom times and long decompression, scuba dives to these depths are generally only done for deep cave exploration or as record attempts.
The difficulties involved in ultra-deep diving are numerous. Although commercial and military divers[ citation needed ] often operate at those depths, or even deeper, they are surface supplied. All of the complexities of ultra-deep diving are magnified by the requirement of the diver to carry (or provide for) their own gas underwater. These lead to rapid descents and "bounce dives". This has led to extremely high mortality rates amongst those who practice ultra-deep diving.[ citation needed ] Notable ultra-deep diving fatalities include Sheck Exley, John Bennett, Dave Shaw and Guy Garman. Mark Ellyatt, Don Shirley and Pascal Bernabé were involved in serious incidents and were fortunate to survive their dives. Despite the extremely high mortality rate, the Guinness World Records continues to maintain a record for scuba diving [25] (although the record for deep diving with compressed air has not been updated since 1999, given the high accident rate). Amongst those who do survive significant health issues are reported. Mark Ellyatt is reported to have suffered permanent lung damage; Pascal Bernabé (who was injured on his dive when a light on his mask imploded [29] ) and Nuno Gomes reported short to medium term hearing loss. [30] [ unreliable source? ]
Serious issues that confront divers engaging in ultra-deep diving on self-contained breathing apparatus include:
In addition, "ordinary" risks like size of gas reserves, hypothermia, dehydration and oxygen toxicity are compounded by extreme depth and exposure and long in-water decompression times. Some technical diving equipment is simply not designed for the greater pressures at these depths, and reports of key equipment (including submersible pressure gauges) imploding are not uncommon.[ citation needed ]
Name | Location | T | A | Depth | Year |
---|---|---|---|---|---|
Ahmed Gabr [24] [32] [33] | Dahab, Egypt | OW | OC | 332 m (1,090 ft) | 2014 |
Nuno Gomes [28] [34] [35] | Dahab, Egypt | OW | OC | 318 m (1,040 ft) | 2005 |
Jarek Macedoński [23] | Lake Garda, Italy | OW | CCR | 316 m (1,040 ft) | 2018 |
Mark Ellyatt [36] | Phuket Island, Thailand | OW | OC | 313 m (1,030 ft) | 2003 |
Xavier Méniscus [37] | Font Estramar, France | C | CCR | 312 m (1,024 ft) | 2024 |
John Bennett [38] [nb 6] | Puerto Galera, Philippines | OW | OC | 308 m (1,010 ft) | 2001 |
Frédéric Swierczynski [39] | Font Estramar, France | C | CCR | 308 m (1,010 ft) | 2023 |
Krzysztof Starnawski [40] | Lake Garda, Italy | OW | CCR | 303 m (994 ft) | 2018 |
Will Goodman [22] | Gili Trawangan, Indonesia | OW | CCR | 290 m (951 ft) | 2014 |
Xavier Méniscus [41] | Font Estramar, France | C | CCR | 286 m (938 ft) | 2019 |
Nuno Gomes [28] [42] | Boesmansgat, South Africa | C | OC | 283 m (928 ft) | 1996 |
Krzysztof Starnawski [43] | Dahab, Egypt | OW | CCR | 283 m (928 ft) | 2011 |
Jim Bowden [44] | Zacatón, Mexico | C | OC | 282 m (925 ft) | 1994 |
Krzysztof Starnawski [45] [46] | Lake Viroit, Albania | C | CCR | 278 m (912 ft) | 2016 |
Han Ting | GuangXi, China | C | CCR | 277 m (909 ft) | 2023 |
Gilberto de Oliveira [28] [47] | Lagoa Misteriosa, Brazil | C | OC | 274 m (899 ft) | 2002 |
Nuno Gomes [28] | Dahab, Egypt | OW | OC | 271 m (889 ft) | 2004 |
David Shaw [28] [48] [nb 6] | Boesmansgat, South Africa | C | DR | 271 m (889 ft) | 2004 |
Frédéric Swierczynski | Mescla, France | C | CCR | 267 m (876 ft) | 2016 |
Pascal Bernabé [28] | Corsica, France | OW | OC | 266 m (873 ft) | 2005 |
Sheck Exley [28] [49] [nb 6] | Nacimiento del Mante, Mexico | C | OC | 265 m (869 ft) | 1989 |
Krzysztof Starnawski [50] [51] | Hranice Abyss, Czechia | C | CCR | 265 m (869 ft) | 2015 |
Sheck Exley [28] [44] [nb 6] | Zacatón, Mexico | C | OC | 264 m (866 ft) | 1989 |
Luca Pedrali [52] | Lake Garda, Italy | OW | CCR | 264 m (866 ft) | 2017 |
Sheck Exley [28] [44] [nb 6] | Boesmansgat, South Africa | C | SCUBA | 263 m (863 ft) | 1993 |
Xavier Méniscus [53] | Font Estramar, France | C | CCR | 262 m (860 ft) | 2015 |
Mark Ellyatt [ citation needed ] | Phuket Island (?), Thailand | OW | OC | 260 m (853 ft) | 2003 |
Qian Chen [54] | Daxing Spring, China | C | CCR | 258 m (846 ft) | 2023 |
John Bennett [38] [nb 6] | Puerto Galera, Philippines | OW | OC | 254 m (833 ft) | 2000 |
Michele Geraci [55] | Bordighera, Italy | OW | OC | 253 m (830 ft) | 2014 |
Jordi Yherla [56] | Font Estramar, France | C | CCR | 253 m (830 ft) | 2014 |
Nuno Gomes [28] | Boesmansgat, South Africa | C | OC | 252 m (827 ft) | 1994 |
Don Shirley [57] | Boesmansgat, South Africa | C | CCR | 250 m (820 ft) | 2005 |
Wacław Lejko [58] [59] [nb 6] | Lake Garda, Italy | OW | OC | 249 m (817 ft) | 2017 |
Xavier Méniscus [60] | Font Estramar, France | C | CCR | 248 m (814 ft) | 2013 |
Karen van den Oever [61] | Boesmansgat, South Africa | C | OC | 246 m (807 ft) | 2022 |
Xavier Méniscus | Goul de la Tannerie, France | C | CCR | 246 m (807 ft) | 2023 |
C.J. Brossett [62] | Gulf of Mexico | OW | OC | 245 m (804 ft) | 2019 |
Richard Harris, Craig Challen [63] | Pearse Resurgence, New Zealand | C | CCR | 245 m (804 ft) | 2020 |
Frédéric Swierczynski [64] [65] | Red Lake, Croatia | C | CCR | 245 m (804 ft) | 2017 |
Guy Garman [66] [nb 6] | St. Croix, U.S. Virgin Islands | OW | OC | 244 m (800 ft) | 2015 |
Dariusz Wilamowski [67] | Lake Garda, Italy | OW | OC | 243 m (797 ft) | 2012 |
Xavier Méniscus | Goul de la Tannerie, France | C | CCR | 243 m (797 ft) | 2019 |
Alexandre Fox | Goul de la Tannerie, France | C | CCR | 242 m (794 ft) | 2017 |
Jim Bowden [68] | Zacatón, Mexico | C | OC | 240 m (800 ft) | 1993 |
Xavier Méniscus | Goul de la Tannerie, France | C | CCR | 240 m (787 ft) | 2014 |
Pascal Bernabé [69] | Fontaine de Vaucluse, France | C | OC | 240 m (787 ft) | 1997 |
A severe risk in ultra-deep air diving is deep water blackout, or depth blackout, a loss of consciousness at depths below 50 metres (160 ft) with no clear primary cause, associated with nitrogen narcosis, a neurological impairment with anaesthetic effects caused by high partial pressure of nitrogen dissolved in nerve tissue, and possibly acute oxygen toxicity. [70] The term is not in widespread use at present, as where the actual cause of blackout is known, a more specific term is preferred. The depth at which deep water blackout occurs is extremely variable and unpredictable. [71] Before the popular availability of trimix, attempts were made to set world record depths using air. The extreme risk of both narcosis and oxygen toxicity in the divers contributed to a high fatality rate in those attempting records. In his book, Deep Diving, Bret Gilliam chronicles the various fatal attempts to set records as well as the smaller number of successes. [72] From the comparatively few who survived extremely deep air dives:
Depth [nb 7] | Year | Name | Location | E | Comment |
---|---|---|---|---|---|
94 m (308 ft) | 1947 | Frédéric Dumas [72] | Mediterranean Sea | OW | A member of the GRS (Groupement de Recherches Sous-marines, Underwater Research Group headed by Jacques Cousteau). |
100 m (330 ft) | 1957 | Eduard Admetlla [73] | Isla de Las Palomas | OW | Head of the Underwater Section of the «Submarine Research and Recovery Centre» |
102 m (335 ft) | 1969 | Frank Salt [72] | Chinhoyi Caves | C | |
106 msw (345 fsw) | 1988 | Marty Dunwoody [72] | Bimini | OW | Women's deep dive record |
107 msw (350 fsw) | 1961 | Hal Watts [72] | Florida | OW | |
109 msw (355 fsw) | 1961 | Jean Clarke Samazen [72] | Florida | OW | |
110 msw (360 fsw) | 1965 | Tom Mount, Frank Martz [72] | Florida | OW | |
120 msw (390 fsw) | 1965 | Hal Watts, A.J. Muns [72] | Florida | OW | |
126 m (415 ft) | 1970 | Hal Watts [72] | Mystery Sink | C | |
131 m (430 ft) | 1959 | Ennio Falco, Alberto Novelli, Cesare Olgiai | Gulf of Naples | OW | Employing the Pirelli Explorer, "Maior" model, a two-stage regulator (patented by Novelli and Buggiani) equipped with a lung bag and soda lime filter for CO2 removal, in order to reuse the exhaled air. Only two of the three divers managed to reach the depth in a certified way: Novelli, the organizer of the event and inventor of the regulator, forgot to punch the plate for proving the descent. [74] |
134 msw (437 fsw) | 1968 | Neal Watson, John Gruener [72] [75] | Bimini | OW | |
135 msw (440 fsw) | 1971 | Ann Gunderson [72] [nb 6] | Bahamas | OW | Women's deep dive record |
139 msw (452 fsw) | 1990 | Bret Gilliam [72] | Roatán | OW | Unusually, Gilliam remained largely functional at depth and was able to complete basic maths problems and answer simple questions written on a slate by his crew beforehand. |
142 m (466 ft) | 1971 | Sheck Exley [76] [nb 6] | Andros Island | OW | Exley was only supposed to go down to 91 m (299 ft) in his capacity as a safety diver (although he had practised several dives to 120 m (390 ft) in preparation), but descended to search for the dive team after they failed to return on schedule. Exley almost made it to the divers, but was forced to turn back due to heavy narcosis and nearly blacking out. |
146 msw (475 fsw) | 1993 | Bret Gilliam [72] | EL Salvador | OW | Again, Gilliam reported no effects from narcosis or oxygen toxicity. |
150 msw (490 fsw) | 1994 | Dan Manion [72] | Nassau | OW | 155 msw (506 fsw) claimed, but not officially recognised. [77] Manion reported he was almost completely incapacitated by narcosis and has no recollection of time at depth. [28] |
156 m (512 ft) | 1999 | Mark Andrews [20] | Puerto Galera, Philippines | OW | At the maximum depth of 156.4 metres (513 ft) Andrews lost consciousness, his deep support diver John Bennett (on mixed gas), inflated his BC to initiate his ascent. While ascending he regained consciousness. |
E Environment: OW = Open water, C = Cave |
In deference to the high accident rate, the Guinness World Records have ceased to publish records for deep air dives, after Manion's dive. [28]
Nitrox refers to any gas mixture composed of nitrogen and oxygen that contains less than 78% nitrogen. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness .The two most common recreational diving nitrox mixes are 32% and 36% oxygen, which have maximum operating depths of about 110 feet and 95 feet (29 meters respectively.
Narcosis while diving is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gases at high partial pressure. The Greek word νάρκωσις (narkōsis), "the act of making numb", is derived from νάρκη (narkē), "numbness, torpor", a term used by Homer and Hippocrates. Narcosis produces a state similar to drunkenness, or nitrous oxide inhalation. It can occur during shallow dives, but does not usually become noticeable at depths less than 30 metres (98 ft).
Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.
Heliox is a breathing gas mixture of helium (He) and oxygen (O2). It is used as a medical treatment for patients with difficulty breathing because this mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs. It is also used as a breathing gas diluent for deep ambient pressure diving as it is not narcotic at high pressure, and for its low work of breathing.
Technical diving is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to a greater risk of serious injury or death. Risk may be reduced via appropriate skills, knowledge, and experience. Risk can also be managed by using suitable equipment and procedures. The skills may be developed through specialized training and experience. The equipment involves breathing gases other than air or standard nitrox mixtures, and multiple gas sources.
Diving medicine, also called undersea and hyperbaric medicine (UHB), is the diagnosis, treatment and prevention of conditions caused by humans entering the undersea environment. It includes the effects on the body of pressure on gases, the diagnosis and treatment of conditions caused by marine hazards and how relationships of a diver's fitness to dive affect a diver's safety. Diving medical practitioners are also expected to be competent in the examination of divers and potential divers to determine fitness to dive.
Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The name scuba is an anacronym for "Self-Contained Underwater Breathing Apparatus" and was coined by Christian J. Lambertsen in a patent submitted in 1952. Scuba divers carry their own source of breathing gas, usually compressed air, affording them greater independence and movement than surface-supplied divers, and more time underwater than free divers. Although the use of compressed air is common, a gas blend with a higher oxygen content, known as enriched air or nitrox, has become popular due to the reduced nitrogen intake during long or repetitive dives. Also, breathing gas diluted with helium may be used to reduce the effects of nitrogen narcosis during deeper dives.
Hydreliox is an exotic breathing gas mixture of hydrogen, helium, and oxygen. For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.
Argox is the informal name for a scuba diving breathing gas consisting of argon and oxygen. Occasionally the term argonox has been used to mean the same mix. The blend may consist of varying fractions of argon and oxygen, depending on its intended use. The mixture is made with the same gas blending techniques used to make nitrox, except that for argox, the argon is added to the initial pure oxygen partial-fill, instead of air.
Peter B. Bennett was the founder and a president and CEO of the Divers Alert Network (DAN), a non-profit organization devoted to assisting scuba divers in need. He was a professor of anesthesiology at Duke University Medical Center, and was the Senior Director of the Center for Hyperbaric Medicine and Environmental Physiology at Duke. Bennett is recognized as a leading authority on the effects of high pressure on human physiology.
Latent hypoxia is a condition where the oxygen content of the lungs and arterial blood is sufficient to maintain consciousness at a raised ambient pressure, but not when the pressure is reduced to normal atmospheric pressure. It usually occurs when a diver at depth has a lung gas and blood oxygen concentration that is sufficient to support consciousness at the pressure at that depth, but would be insufficient at surface pressure. This problem is associated with freediving blackout and the presence of hypoxic breathing gas mixtures in underwater breathing apparatus, particularly in diving rebreathers.
Hydrox, a gas mixture of hydrogen and oxygen, is occasionally used as an experimental breathing gas in very deep diving. It allows divers to descend several hundred metres. Hydrox has been used experimentally in surface supplied, saturation, and scuba diving, both on open circuit and with closed circuit rebreathers.
Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.
Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.
Human physiology of underwater diving is the physiological influences of the underwater environment on the human diver, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. It, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. Several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. All of these may affect diver performance and safety.
The history of scuba diving is closely linked with the history of the equipment. By the turn of the twentieth century, two basic architectures for underwater breathing apparatus had been pioneered; open-circuit surface supplied equipment where the diver's exhaled gas is vented directly into the water, and closed-circuit breathing apparatus where the diver's carbon dioxide is filtered from the exhaled breathing gas, which is then recirculated, and more gas added to replenish the oxygen content. Closed circuit equipment was more easily adapted to scuba in the absence of reliable, portable, and economical high pressure gas storage vessels. By the mid-twentieth century, high pressure cylinders were available and two systems for scuba had emerged: open-circuit scuba where the diver's exhaled breath is vented directly into the water, and closed-circuit scuba where the carbon dioxide is removed from the diver's exhaled breath which has oxygen added and is recirculated. Oxygen rebreathers are severely depth limited due to oxygen toxicity risk, which increases with depth, and the available systems for mixed gas rebreathers were fairly bulky and designed for use with diving helmets. The first commercially practical scuba rebreather was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London. His self contained breathing apparatus consisted of a rubber mask connected to a breathing bag, with an estimated 50–60% oxygen supplied from a copper tank and carbon dioxide scrubbed by passing it through a bundle of rope yarn soaked in a solution of caustic potash. During the 1930s and all through World War II, the British, Italians and Germans developed and extensively used oxygen rebreathers to equip the first frogmen. In the U.S. Major Christian J. Lambertsen invented a free-swimming oxygen rebreather. In 1952 he patented a modification of his apparatus, this time named SCUBA, an acronym for "self-contained underwater breathing apparatus," which became the generic English word for autonomous breathing equipment for diving, and later for the activity using the equipment. After World War II, military frogmen continued to use rebreathers since they do not make bubbles which would give away the presence of the divers. The high percentage of oxygen used by these early rebreather systems limited the depth at which they could be used due to the risk of convulsions caused by acute oxygen toxicity.
The following outline is provided as an overview of and topical guide to underwater diving:
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